1. Field of the Invention
The present invention relates to the surface treatment of large metal parts for the purpose of deoxidizing them and/or of cleaning them, with reference especially to the metal sheets as they are presented when leaving a rolling mill, in order to modify its [sic] surface properties while maintaining its [sic] volume properties.
Such a part generally has, on the surface, irregularities having a size of about 0.1 to 0.5 xcexcm, lubrication residues and traces of oxidation, which make coatings applied to this surface poorly adherent.
2. Description of the Related Art
In general, the preparation of the surface of a thin sheet for the purpose of subsequently treating it, and in particular for the purpose of increasing the adhesion of coatings applied to this surface, comprises a necessary step of cleaning and deoxidizing the sheet.
To do this, a widely-used surface treatment technique consists in immersing the part to be treated in an acid or base chemical bath or in an organic solvent.
Although this type of technique makes it possible to treat parts having relatively large dimensions, and to do so at a relatively high rate, it is accompanied by the emission of environmentally harmful effluents.
It has also been proposed, for example in document U.S. Pat. No. 5,376,223 or else in the article by Sakamoto et al., published in the Japanese Journal of Applied Physics in May 1980 (page 839), that use be made of a hydrogen plasma under ECR (Electron Cyclotron Resonance) conditions to carry out surface treatments especially on silicon.
It is known that ECR is one particular way of coupling the energy of an electromagnetic field into the electrons of a plasma in order to sustain the latter.
Both the abovementioned documents are representative of the conventional method used for producing an ECR plasma reactor, which method consists in injecting microwaves from a waveguide into a vacuum chamber through a dielectric window (by way of illustration, chamber 1, waveguide 5 and window 4 in FIG. 1 of the abovementioned US document).
In-depth studies by the Applicant in this field have shown that such a configuration is not satisfactory for the application intended according to the present invention, i.e. for surface treatment allowing the surface properties of large sized metal parts, particularly metal sheets, to be modified.
This is because a static magnetic field is created in the relatively extensive volume region of the chamber by induction coils (referenced 2 in FIG. 1 already mentioned). It therefore appears that the region in which the plasma is created by energy transfer from the microwave field to the electrons necessarily has a diameter limited by the transverse dimensions of the waveguide, even though downstream of the coils the plasma diffuses toward the substrate by extending radially, but nevertheless with the knowledge that this diffusion is, of course, accompanied by a radial density gradient in the plasma.
It should also be emphasized that the magnetic field is not suddenly interrupted outside the coil (2 in the figure already mentioned of the US document), but has a magnetic field gradient which extends beyond the inter-coil space toward the substrate and which has the effect of accelerating the ions in the plasma toward the substrate, this being all the more so when at a large distance from the axis, therefore resulting, as will be understood, in a high radial inhomogeneity.
These studies therefore seem to demonstrate that it is unrealistic to treat substrates having a surface area greater than 200 or 300mm [suc] using such ECR reactors.
The object of the invention is to alleviate these drawbacks and to provide a method for the surface treatment of a metal part which can be carried out rapidly, on large areas and near the places where the parts undergo subsequent treatment.
The subject of the invention is therefore a method for the surface treatment of a metal part for the purpose of deoxidizing it and/or cleaning it, characterized in that it comprises the following steps:
filling a sealed chamber, in which the part to be treated is placed, with a low-pressure reducing gas mixture;
creating a static magnetic field in a region of the chamber separate from the region in which the part to be treated is located; and exciting the gas mixture by means of an electromagnetic wave injected into the chamber so as to generate a treatment plasma in the gas, the intensity of the static magnetic field corresponding to an electron cyclotron resonance established in the chamber in a distributed manner.
As will have been understood, the method according to the invention makes it possible to obtain, depending on the initial state of the part, deoxidization and cleaning treatments, this being so by the combination of a static magnetic field created in one region of the chamber separate from that in which the part to be treated lies and of a plasma excitation of the gas mixture allowing xe2x80x9cdistributed ECR (Electron Cyclotron Resonance)xe2x80x9d conditions to be established.
Reference may be made to this literature from the 1980s and 1990s relating to such plasma excitations, including especially the following two articles:
the article by M. Pichot et al., published in Review of Scientific Instruments in 1988, July, Vol. 59, p 1072; and
the article by J. Pelletier et al., published in Thin Solid Films in 1994, Vol. 241, p 240.
As will be clearly apparent to those skilled in the art, the method according to the invention adopts xe2x80x9clow pressurexe2x80x9d conditions, which must be understood to mean pressure conditions allowing actual excitation of the plasma under electron cyclotron resonance (ECR) conditions, whereas at higher pressures the only effect of the magnetic field would be the confinement of the plasma by the multipole magnetic field (magnetron regime). The in-depth studies by the Applicant have shown that this magnetron regime, well known to those skilled in the art, gives results which are substantially lacking as regards the surface treatment application envisioned.
Advantageously, the pressure will therefore be within a range extending from 0.5 to 10 mTorr, and even more preferably within a range extending from 1 to 10 mTorr.
Likewise, the frequency of the incident electromagnetic wave adopted will take into account, on the one hand, the desired ECR regime and also the nature of the gas to be excited without forgetting the practical and technical contingencies of commercial availability and cost (frequencies for domestic use), and those pertaining to the international regulations on radiocommunications.
Taking all these factors into account, it will therefore be preferable to use a frequency of 2.45 GHz, or else a frequency of 5.85 GHz.
It should be understood from the notion of xe2x80x9cdistributed electron cyclotron resonancexe2x80x9d according to the invention that the excitation means used comprise at least one field applicator of tubular overall shape connected to a microwave source, each applicator being equipped with means for creating a static magnetic field in the vicinity of the applicator, over a length corresponding substantially to the entire length of the applicator, with an intensity corresponding to electron cyclotron resonance.
The method according to the invention may furthermore include one or more of the following characteristics, taken separately or in any of the technically possible combinations:
the electromagnetic wave is injected by means of at least one applicator of tubular overall shape connected to a microwave source, in which applicators permanent magnets are placed;
the applicator or applicators have a length at least equal to one of the dimensions of the metal part to be treated;
the gas mixture includes hydrogen;
the part to be treated consists of a steel sheet running over a substrate holder;
the method furthermore includes a step consisting in applying a high-frequency electric field to the substrate holder so as to bias both the latter and the part to be treated.
Further characteristics and advantages will emerge from the following description, given solely by way of example and with reference to the appended drawings in which.